1. Field of the Invention
The present invention relates generally and according to a first aspect to methods of separating a dry feed gas mainly comprising methane, ethane and propane, typically natural gas, and in a second aspect to industrial installations and equipment allowing these methods to be carried out.
2. Description of the Art
More precisely, the invention relates, according to a first aspect, to a method of separating a dry feed gas, mainly comprising methane, ethane, and propane, into a first, relatively more volatile product, called treated gas, and a second, relatively less volatile product, called C2 plus fraction, comprising:    (i) an operation for cooling the feed gas, turning it into a cooled gas,    (ii) an operation for separating and treating the cooled gas produced by operation (i), this cooled gas being separated into a first, essentially liquid bottom stream and a first, essentially gaseous top stream, the first bottom stream then being at least partially expanded in order to form a first cooled bottom stream, the first top stream being separated into a main stream and a secondary stream, the main stream being expanded in a turbine in order to form an expanded main stream, and the secondary stream being cooled in an exchanger, then expanded in order to form an expanded secondary stream,    (iii) a distillation operation in a distillation device producing a second top stream and a second bottom stream, the distillation device being fed by at least part of the expanded main stream, by at least part of the cooled bottom stream, and by at least part of the expanded secondary stream, the cooled bottom stream being at a relatively less cold temperature than the expanded main stream and the expanded secondary stream being at a relatively colder temperature than the expanded main stream, the second top stream cooling the secondary stream in the exchanger then, after reheating and a plurality of compression and cooling stages, forming the first product, with the second bottom stream, after compression and reheating, forming the second product.
According to a second aspect, the invention relates to an installation for separating a dry feed gas, mainly comprising methane, ethane and propane, into a first, relatively more volatile product, called treated gas, and a second, relatively less volatile product, called C2 plus fraction, comprising:    (i) means for cooling the feed gas, turning it into a cooled gas,    (ii) means for separating and treating the cooled gas produced by stage (i), this cooled gas being separated into a first, essentially liquid bottom stream and a first, essentially gaseous top stream, the first bottom stream then being at least partially expanded in order to form a first cooled bottom stream, the first top stream being separated into a main stream and a secondary stream, the main stream being expanded in a turbine in order to form an expanded main stream, and the secondary stream being cooled in an exchanger, then expanded in order to form an expanded secondary stream,    (iii) a distillation device producing a second top stream and a second bottom stream, the distillation device being fed by at least part of the expanded main stream, by at least part of the cooled bottom stream, and by at least part of the expanded secondary stream, the cooled bottom stream being at a relatively less cold temperature than the expanded main stream and the expanded secondary stream being at a relatively colder temperature than the expanded main stream, the second top stream cooling the secondary stream in the exchanger then, after reheating and a plurality of compression and cooling stages, forming the first product, the second bottom stream, after compression and reheating, forming the second product.
This method and the installation which carries it out are known from the prior art, in particular from U.S. Pat. No. 4,157,904. This patent discloses several methods and their corresponding installations having the characteristics described above, these methods also providing for mixing part of the first bottom stream with the secondary stream prior to cooling, expansion and feeding into the distillation device.
The distillation device used by these methods is formed by a distillation column. The secondary stream is introduced at the top of the column and acts as a reflux and the main flux is introduced at an intermediary stage. The cooled first bottom stream is introduced at a lower stage to the main stream.
The top of the column, between the introduction stage of the main stream and the introduction stage of the secondary stream, acts as an extraction zone for C2 and higher hydrocarbons from the main stream, and the bottom of the column, below the main stream introduction stage, acts as a methane-removal zone.
The ethane and propane extraction efficiencies can be increased by lowering the temperature profile of the column. This requires a lot of energy if the power of the cooling cycle used to cool the feed gas is simply increased.
Another way of lowering this profile is to expand the streams feeding the distillation column to a larger extent, whereby these streams are cooled but the operating pressure of the column is also reduced. The power required for recompressing the first product will therefore increase.
U.S. Pat. No. 4,157,904 proposes plans allowing this profile to be lowered by optimising energy efficiency, mainly by mixing part of the first bottom stream with the secondary stream prior to cooling, expansion and feeding into the distillation device, which, as a result of the physico-chemical properties of these streams, allows lower temperatures for feeding the distillation column to be reached without having an adverse effect on the operating pressure.
By contrast, the reflux, formed by the mixture of part of the first bottom stream and the secondary stream, is richer in C2 and higher hydrocarbons than the secondary stream alone, which has an adverse effect on the extraction of C2 and higher hydrocarbons from the main stream in the top zone of the column.